Page 224 - Know and Understand Centrifugal Pumps

Failure Analysis of Mechanical Seals

pH so that the boiler tubes won’t corrode. Next the treatment process
removes oxygen, which prevents internal boiler wetted parts from
rusting. Third, the treatment process removes minerals from the water
so that mineral scale won’t form on the boiler tubes, insulating them,
and causing the boiler to lose efficiency. After raw water has been
treated with chemicals to scavenge the oxygen, remove or neutralize
minerals, and control the pH, the raw water becomes make-up water.
The treatment chemicals and the treated water will need Ethylene
Propylene O-rings on the mechanical seals, instrumentation, valves,
connections and fittings.

If the boiler is a high-pressure boiler, the boiler’s discharge valves, and
instrumentation fittings may need perfluorocarbon O-rings for
temperatures above 300 degrees. The high-pressure boiler feed water
pump may need these high temperature O-rings in the mechanical seals
because of the high frictional heat generated by the seal faces. If the DA
(deaerator) tank is sealed and pressurized to hold the hot water from
flashing, it may need these high temperature O-rings.

If the boiler burns propane, natural gas, or fuel oil, then you’ll need
fluorocarbon O-rings on your fuel lines, valves, instrumentation and
fittings.
Who would have thought that an industrial boiler would need up to
four different O-ring compounds just to heat some water?

When an O-ring comes out of service, it should be inspected for signs of
damage and degradation. These could be:

Chemical attack
Because the O-ring comes into contact with the fluid, the O-ring’s
rubber compound must be chemically compatible with the fluid.
Chemically compatible means that the O-ring will resist the chemical
without degradation. If the O-ring is not chemically compatible with
the pumped liquid, it may swell, harden, dry and crack, soften, or even
dissolve depending on the nature of the chemical attack. The surface of
the O-ring may form blisters, scale, or form fissures and cracks. The
cause of these symptoms generally is chemical attack. The attack may
come from the pumped liquid, or from the barrier fluid or external
flush in the case of installed environmental controls. Environmental
controls are discussed later in this chapter. Too much heat in the system
may present the same evidence. You must be familiar with the different
O-ring compounds used as secondary seals in mechanical seals, and all
instrumentation, connections and fittings. You must know the
temperature limits (upper and lower) and the chemicals they are
compatible with.